ok, most likely there is some buildup in the pilot jet. this is a jet that is in the float chamber of the carb it is responsible for idle speeds and aids in starting as well. this jet plugs very easily. and is known to start to do this as little as 3 weeks or so depending on fuel quality. but first, just to make sure you are getting fuel to the carb, if you look at the bottom of the carb there is a drain screw for the float bowl. go ahead and unscrew it and see that fuel flows out. if none does, then you may have a blockage in the fuel line, fuel tank, or the float valve in the carb may have stuck closed. again that is a condition from buildup in the carb from sitting. but most likely you are dealing with a plugged jet. if you do get fuel flow, then the problem is in the carb. also if you notice where the choke plunger goes into the carb, shoot carb cleaner in the hole the choke plunger needle seats into. ensure that carb cleaner comes back out the tube into the float chamber as well. this can clog just as easily, and cause a no start.
you will need to remove the carb from the atv, and remove the bottom float bowl. at that point you will see tubes that come down into that float area. those have fuel jets in them. the smallest orifice one is the pilot jet. that is one that you will need to concentrate on. take a small gauge wire. something compared to a wire brush bristle, and clean through that jet as well as with some aresol carb cleaner. oh and you may want to removed it from the carb to do this so dirt doesnt get internally intot he carb. but clean that jet out very well. the smallest amount of buildup makes a big difference. once that is clean shoot some carb cleaner in the hole that the jet goes into. and see that carb cleaner exits out into the throat of the carb through a small hole. just to make sure the way is clear.
doing these things will get your atv back to starting on its own again. it will also help if you clean the main jet(the larger jet) as well just incase this has started to get some buildup as well.
hope this helps. any questions about this please ask, ill be happy to help. thank you.